Shark Serengeti: Ocean predators have diversity hot spots

The first search for oceanic spots of exceptional diversity in predators has turned up marine versions of the teeming Serengeti plains and Amazon rain forests.

Records from fishing boats highlight four areas showing unusual diversity in sharks, tuna, billfishes, and other big predators, says Boris Worm of the Institute for Marine Science in Kiel, Germany. Those “major hot spots” are in waters off the east coast of Florida, south of Hawaii, off the Great Barrier Reef, and near Australia’s Lord Howe Island, Worm and his colleagues report.

An overall pattern shows peak diversity in middle latitudes near prominent underwater geographic features, the researchers contend in an upcoming Proceedings of the National Academy of Sciences.

The new work “brings up some pretty major issues in conservation,” comments shark ecologist Mark Meekan of the Australian Institute of Marine Science near Darwin.

The slow reproduction of many large fish renders them especially vulnerable to overfishing when they cluster, he says. Yet preserving a hot spot or two may not adequately protect these creatures, which migrate long distances.

Worm traces his interest in hot spots to work begun in the 1980s highlighting biodiversity centers on land. Norman Myers of the University of Oxford in England and others have inspired widespread efforts to protect such locations as a biggest-bang-for-the-buck conservation strategy (SN: 8/17/96, p. 101: https://www.sciencenews.org/sn_arch/8_17_96/fob2.htm).

The strategy has been slow to get its feet wet, though. One 1999 study located zones of high zooplankton diversity, but not until last year did researchers designate the top-10 hot spots for coral reef biodiversity (SN: 2/16/02, p. 100: Available to subscribers at Biodiversity Hot Spots: Top 10 sea locales make sobering list).

To consider the top of the marine food chain, Worm and his colleagues turned to fishing records, as they have also done to document worldwide declines in fish populations (SN: 7/26/03, p. 59: Catch Zero). Both the United States and Australia require scientific observers to sail with selected commercial long-line fishing vessels. The observers record the species snagged by the strings of hundreds of hooks.

Analyzing data from the 1990s, the researchers picked out hot spots of predator diversity, where for every 50 creatures caught on a hook, at least 12 predator species showed up, on average. This underwater diversity tended to bloom between 20 to 30 degrees latitude both in the northern and southern hemispheres. Tropical and temperate species mingle there.

Also, Worm says, diversity spikes where big reefs, seamounts, or other features roil the water and kick nutrients up into sunlit zones where many organisms can use them.

Worm and his colleagues used a computer model to forecast effects of hot spot protection on commercial fishing. Safeguarding the hot spot off Florida looks particularly promising, they say, because it’s not especially rich in commercial species.

Meekan welcomes the work not just for its conservation implications but for illuminating how big predators survive in the relatively food-poor waters of the open ocean. “What they’re probably doing is trekking like camels from oasis to oasis,” he says.

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